Rotary optical encoders are often used to measure the angular position of a motor shaft. Presently known rotary optical encoders devices employ optical detectors to monitor the motion of a disc that is attached to the motor shaft. The optical detectors and an associated light source are mounted within a read station, or head, which are fixed or locked in a stationary position with respect to the encoder housing. Typically the disc has a series of light and dark lines encoded on its surface which are illuminated in the region of the optical detectors by the light source. The portion of the disc demarcated is referred to as the track. As the illuminated disc rotates beneath the detectors, the amount of illumination impinging on the optical detector surfaces fluctuates. The amount of shaft rotation is determined by counting the number of intensity fluctuation sensed by the detector. Since the angular width of the lines is known at a particular radius on the disc, the arc length viewed by the head and the associated angular rotation of the disc may be determined.
There are fundamentally three optical arrangements which can be used. The first arrangement is a transmissive scheme wherein opaque lines are encoded on a transparent disc. The light source is placed opposite the optical detectors with the disc rotating between the two. In order to enhance contrast, a mask between the disc and the detectors may be employed and collimating optics to form a proper illumination beam may be used.
A second arrangement is to place the detectors and the light source on the same side of the disc. In this reflective scheme, the disc is constructed in a way that the disc reflects varying amounts of light back to the optical detectors. A variation on this arrangement involves applying the principles of interferometry. The disc is grooved so that the stripes on the disc lie in two planes distanced by a fraction of a wavelength of light. A third arrangement is similar, but is based upon principles of diffraction and interferometry. In this approach the disc is constructed so that it acts as a diffraction grating.
One limitation of conventional rotary encoders is their sensitivity to eccentricities in the disc or shaft relative to the detectors. These cause the radius from the center of rotation to the portion of the track being observed by the read stations to vary. In order to properly interpret the fluctuations in illumination in terms of arc length, one must have knowledge of the instantaneous radius throughout the sweep of the disc. Otherwise, the calculated or perceived rotation will deviate from the actual rotation. Given that the conventional rotary encoder has fixed heads, this deviation cannot be accounted for by the individual heads. In order to minimize sensitivity to this phenomenon, multiple heads are often used and the detected signals are averaged. However if the encoder track being monitored should deviate to such an extent that it lies outside the detector's field of view, no motion of the disc will be detected. Thus, eccentricity has a significant effect on accuracy.
Another limitation of conventional rotary and linear encoders is their sensitivity to variations in the distance between the optical reading head lens and the rotary disc or linear scale below the head. These variations, caused by disc/scale deviations from the tight surface flatness requirements and/or assembly flatness, restrict the ability of keeping the reading head lens appropriate distance from the encoder disc/scale, i.e., the lens depth of focus (DOF), and increase the cost of assembling and/or maintaining the encoder.
Therefore, it would be advantageous to provide a solution that overcomes at least the deficiencies noted above.